Hammer Warriors Overwrite Stability/Evasion?
It sounds like a bug. None of the Hammer skills remove boons, but some of them (Knockdown, the 5 skill, and the F1 burst skill) do stun. I think it’s probably an error on your end where the stability isn’t actually negating stuns.
That or you might be getting hit by the 4 skill, which interrupts and knocks back but isn’t a stun, and might not be negated by stability.